Pissed off = angry
Pissed up = drunk (a piss-up is an opportunity to drink heavily)

So you can see where that comes from.

On the subject of car parts we have 'bonnet' for what in the USA is I believe the 'hood'. Now isn't that just cute and British?

I suppose that in the British Isles and other places where English is spoken we understand more of the Americanisms because of our contact with US films, TV and books. Anyway it's time for me to roll a fag (which is commonspeak here for making a cigarette).